Electronic image-capture devices are typically divided broadly into two types: cameras and scanners. Cameras employ electronic image sensors that have a two-dimensional (i.e., areal) array of photosensitive areas (or “photosites”) that permit an image of a scene to be captured without requiring relative motion between the scene to be captured, the image sensor, and any optical elements used for forming an optical image of the scene on the image sensor. The photosites typically collect photon-induced electrical charge (or “photocharge”) over some time period, and the electrical charge is measured and transformed into the picture elements (pixels) of the captured image. By way of example, the KODAK KAF-8300 is an areal-array image sensor for use in cameras. The KAF-8300 includes a 3326×2504 two-dimensional array of photosites, each of which separately collects photocharge, and which collectively provide 8.3 million pixels in a captured image.
In contrast, scanners typically require relative motion between the scene to be captured and the image sensor, or movement of optical elements used for forming an optical image on the sensor, to sweep the optical image of the scene across the photosensitive areas of the electronic image sensor. Scanners typically employ electronic image sensors that have a one-dimensional (i.e., linear) array of photosensitive areas. Photocharge is allowed to accumulate over some time period in the photosites, and the resulting accumulated charge in each photosite is then measured. This accumulate-and-measure process occurs repetitively during the scanning process, with each iteration forming a single line of pixels in the captured image. In this way, a two-dimensional captured image is formed from successively captured single lines of pixels. In one common scanner variant, multiple linear arrays, each array provided with a color-selective filter, are employed to capture color images. By way of example, the KODAK KLI-4104 is a linear-array image sensor for use in scanners. The KLI-4104 includes four linear arrays: three separate arrays of 4080 10 μm photosites, with each array filtered to capture red, green, or blue light, and a fourth array of 8160 5 μm photosites that are unfiltered to permit capture light over a broad color spectrum.
Scanners are used for capturing images of documents, for capturing images of moving items in an manufacturing plant (for example, canned beverages), for robotic vision (typically employing a polygonal mirror to sweep the scene image across the linear sensor), and for airplane- or satellite-based imaging of the surface of the Earth. In some of these applications the image-capture device may be called a camera, but if the application requires relative motion between the scene and the image sensor (or movement of optical elements to produce an effective motion of the scene across the image sensor) it employs a scanner as the term is used herein.
A time-delay-and-integrate (TDI) sensor is a particular type of electronic image sensor employed in scanners. In contrast to a linear-array sensor, a TDI sensor employs multiple photosites that are used collectively to form each pixel of the captured image. The multiple photosites used for a given pixel are arranged in a column that is aligned with the direction of motion of the optical image across the sensor. In this way, each photosite in the column is presented sequentially with a particular portion of the optical image. The photocharge accumulated in each successive photosite during the time that the portion of the optical image moves over the photosite contributes to the respective pixel in the captured image. In this manner, the TDI sensor increases the photocharge accumulation time for each pixel of the captured image. A typical TDI sensor includes many column-wise photosites arranged in parallel.
A TDI charge-coupled device (CCD) sensor 100 is shown in FIG. 1. The TDI sensor 100 includes multiple integrating CCDs (ICCDs) 102, a readout CCD (RCCD) 104, and a charge-measurement and amplifier circuit 106. (In some descriptions, the ICCDs are called vertical CCDs and the RCCD is called a horizontal CCD.) As an optical image sweeps vertically downward across the ICCDs 102, the charge-shifting mechanism of the ICCDs is employed to move packets of charge downward simultaneously with the movement of the optical image. As a packet of charge moves from the top of an ICCD 102 to the RCCD 104, it travels through multiple photosites and accumulates additional photocharge along the way. When a line of charge packets reaches the RCCD 104, the charge packets are shifted laterally in the RCCD (in the illustrated embodiment) to be individually read out of the sensor 100 by the charge-measurement and amplifier circuit 106. The time required for a charge packet to travel from the top of an ICCD 102 until it enters the RCCD 104 for readout is the exposure time, or integration time, for a given pixel of the captured image. Compared to a conventional linear-array image sensor, a TDI image sensor typically enables significantly increased integration time.
FIG. 2 further illustrates the operation of a TDI image sensor. Light from scene element 202 is collected by optical system 204 in order to produce an optical image 206 on the face of the TDI image sensor. Scene element 202 moves vertically upward with respect to optical system 204 and the TDI image sensor. This causes corresponding optical image 206 to move vertically downward across the surface of the TDI image sensor. Simultaneously with the downward motion of the optical image 206, the ICCDs of the TDI image sensor are clocked downward toward the RCCD of the TDI image sensor. As each line of accumulated packets of photocharge from the ICCDs is clocked vertically into the RCCD and then horizontally out through the TDI image sensor's charge-measurement and amplifier circuit, a line of pixels of the captured image is produced.
If a scene to be captured is sufficiently bright, allowing photocharge to accumulate over the length of the ICCD may cause the accumulated photocharge to exceed the charge capacity of the ICCD for the brightest areas of the scene. To avoid this, one of the horizontal clock lines for the ICCDs may be held in a state to prevent charge packets from above the clock line from continuing below the clock line. For example, horizontal clock line 108 (see FIG. 1) may be used to block charge from the upper 15/16 of the ICCDs 102, effectively reducing the integration time to 1/16 of the potential full integration time. In similar fashion, horizontal clock line 110 reduces integration time to ⅛ of the full integration time, horizontal clock line 112 reduces integration time to ¼ of the full integration time, and horizontal clock line 114 reduces integration time to ½ of the full integration time.
By way of example of the type of image sensor shown in FIG. 1, a TDI image sensor is described in “A High Speed, Dual Output Channel, Stage Selectable, TDI CCD Image Sensor for High Resolution Applications” (Agwani, et al, Proc. SPIE, Vol. 2415, Page 124 (1995)). The device described has 2048 ICCDs, each ICCD consisting of 96 CCD integrating stages, and with the number of integrating stages selectable among 96, 48, 24, 12, and 6 stages. The RCCD is split into two CCDs, with one CCD for even-numbered ICCDs and the other for odd-numbered ICCDs, and with separate charge measurement and amplifier circuits associated with each of the two readout CCDs. The sensor provides captured image pixel lines at up to 14,000 lines per second, and provides a dynamic range of 6000:1.
Although TDI CCD image sensors generally have very high sensitivity due to the long integration times provided by the ICCDs and also have flexibility in integration time by selecting the number of stages of integration employed, there remains a need for greater dynamic range. For example, when capturing images of the Earth's surface, a natural body of water or water standing on the roof of a building may reflect sunlight, while nearby scene elements may be dark or in shadow. In such a situation, the range of light level between the reflected sunlight and the dark areas of the scene may far exceed the 6000:1 dynamic range of a typical CCD TDI image sensor such as the one described above.
One proposed technique for increasing dynamic range in a TDI CCD is described in “An Adaptive Sensitivity™ TDI CCD Sensor” (Chen and Ginosar, Proc. SPIE, Vol. 2950, 45 (1996)). In this sensor, each ICCD is composed of 13 TDI stages, a conditional reset circuit, 4 more TDI stages, another conditional reset circuit, and a final TDI stage before reaching the RCCD. The conditional reset circuits include a charge-measurement amplifier that controls a discharge gate: as each charge packet is clocked through the CCD stage associated with the conditional reset circuit, the amount of charge is measured. If the measured charge exceeds a threshold, the discharge gate is operated to remove the charge from the CCD. In this fashion, the dynamic range of the image sensor is increased: dark areas of the scene do not cause either of the conditional reset circuits in an ICCD to trigger, thereby getting the benefit of the full 13+4+1=18 TDI stages; middle brightness areas of the scene cause the first conditional reset circuit to trigger, but not the second, allowing the use of 4+1=5 TDI stages; and the brightest areas of the scene cause both conditional reset circuits to trigger, thereby using only a single TDI stage to capture those areas of the scene. Effectively this increases the dynamic range of the sensor by a factor of 18, i.e., the difference between 18 TDI stages used for dark areas of the scene and 1 TDI stage used for bright areas of the scene.
However, there are several shortcomings with this approach. First, the conditional reset circuit consumes a significant amount of area, as it includes multiple transistors. Second, a contact must be placed in the CCD stage associated with the conditional reset circuit to permit the measurement of charge, and the contact has the potential for producing dark current or otherwise affecting the charge packet as it passes through the affected CCD stage. Additionally, there is no mechanism for determining from the output whether a particular pixel integrated over the full 18 stages, was reset once and integrated over only 4 stages, or was reset twice and integrated over only a single stage. Therefore, there remains a need to increase the dynamic range of a TDI CCD image sensor while addressing these shortcomings.